Monocrystalline HgCdTe is considered highly suitable for the fabrication of optoelectronic devices operating at high wavelengths (2-40 .mu.m). More particularly, because of the low energy gap between valence and conduction bands, it is suitable for the fabrication of photodetectors for infrared radiation with wavelengths ranging between 8 and 14 .mu.m, when the HgCdTe material contains about 10% by weight of cadmium.
In fact, the energy necessary to produce electron-hole pairs in the semiconductor is only about 0.1 eV for the cited composition.
Optoelectronic devices can thus be fabricated for detecting low-energy radiations, such as, for instance, those emitted by an object at room temperature.
Monodimensional or bidimensional arrays, forming the photosensitive elements of high-performance imaging systems, can be obtained with devices of this kind. Such systems can supply thermal images useful in various applications, by instance illness diagnosis, terrestrial pollution mapping from on board satellites, sighting of objects or people under poor visibility conditions, etc.
As is known high-performance electronic devices can be fabricated using a basic semiconductor material in the form of a single crystal. In fact only in the case of monocrystal does the material possess physical properties which are constant and well defined at every point of its volume. That is why the performances of the devices obtained with such materials may be optimized.
In addition, the single crystal used must have a lattice periodicity which is as perfect as possible in order to avoid undesired reductions in the efficiency of photodetection on the quantum level.
The desired characteristics of such materials can best be obtained by epitaxial growth techniques, which also allow the formation of the broad surfaces necessary to the fabrication of photodetector arrays. Liquid and vapor phase epitaxial growth techniques are well known. The former are the most-widely used at the present time.
Liquid-phase epitaxial growth consists of the deposition on a single-crystal substrate of alloys with compositions depending on those of suitable growth solutions. Starting from solutions where melted tellurium acts as solvent and cadmium and mercury are the solutes, single-crystal layers of the ternary mercury cadmium telluride compound can be deposited on the binary cadmium telluride compound, generally used as substrate. The deposition takes palce by bringing the substrate into contact with the slightly supersaturated solution.
The growth solution has been prepared heretofore in a process and location different from that at which epitaxial growth takes place. The preparation method usually involved sealing in a quartz ampoule convenient quantities of tellurium, cadmium and mercury, homogenizing them at a temperature higher than 500.degree. C. for some tens of hours, and of rapidly quenching them to preserve the homogeneous composition of the liquid. Parts of this alloy were then used as the growth solutions in subsequent cycles of epitaxial deposition of a single-crystal substrate.
Unluckily, all techniques used in growing cadmium mercury mixed tellurides are highly complicated by the strong tendency of mercury to evaporate.
This tendency to evaporate is favored by the need to operate the growth process at temperatures higher than the melt temperature of solvent tellurium, which is about 500.degree. C.
Mercury evaporation causes variations during growth thermal cycles both in the single-crystal solid, and in the solution it is generated by (for liquid-phase techniques).
To overcome this disadvantage, different methods have been suggested to protect the solution composition. A method described in Semiconductors and Semimetals, Academic Press, Vol. 18, 1981, pages 70-84, proposes insertion in a sealed ampoule of both the solution and the substrate, on which the deposition is effected by bringing into contact the solution and solid.
In practice this method has been abandoned owing to the difficulties encountered in controlling growth phases inside the closed ampoule and the impossibility of implementing structures with different composition layers.
According to another method, described by T.C. Harman in papers published in Journal of Electronic Materials, Vol. 9, No. 6, pages 945-961 and Vol. 10, No. 6, pages 1069-1085, the solution and substrate are placed into a reactor with a hydrogen atmosphere at atmospheric pressure and the solution composition is preserved by means of a mercury source inside the reactor itself. The source consists of a mercury bath maintained at a lower temperature than that of the growth solution, but such as to produce a partial mercury vapor pressure equal to that of the solution equilibrium. That is obtained by placing the reactor into a furnace with two zones at different temperatures. This method, according to the literature, requires the solution to be prepared in a phase preceding the growth phase. Preparing the solution in a sealed ampoule can be disadvantageous due to possible contamination of the solution during handling and because the process is lengthened and thus industrially less interesting.